Automobile enthusiasts often make extensive modifications to their cars to enhance performance. Such modifications may often involve replacing the engine and transmission for greater speed and acceleration or simply modifying the existing equipment with custom cams, exhaust headers, intake manifolds and perhaps superchargers. This quest for power is intrinsic to the scene of our American car culture. It is fueled by drag strip competition to some extent, stop light machismo to some extent and a personal need to give one's car some “special” identity. Whatever the motivation, many of the cars given such treatment will have automatic transmissions or mechanical clutch release mechanisms as original equipment.
An automatic transmission is not acceptable to many enthusiasts, who object to inappropriate speed ratio selections and the power losses inherent to automatics. Installation of a clutch pedal from salvage or reproduction parts sources is not a difficult task for any skilled mechanic.
A mechanical linkage is serviceable enough for the average driver, but many enthusiasts prefer a hydraulic clutch release system for more precise control and better feel. Non-standard engine, transmission and exhaust installations may also introduce other clearance issues. For these reasons, mechanical clutch release mechanisms are routinely discarded in favor of hydraulic actuators as part of the overall changes.
In converting a car to use a hydraulic clutch release system, the hydraulic clutch master cylinder is preferably mounted to the sheet metal of the firewall, on the engine side, opposite the clutch pedal, where it can be operated by the newly installed or existing pedal arm for actuation. The firewall sheet metal is generally vertical or somewhat inclined and is also relatively thin, so that some form of reinforcement is required to support and stabilize the master cylinder under operating forces. Pedal arm angle, its pivot axis location and the desired mechanical advantage for pedal force, combine to establish the line-of-action angle of the master cylinder push-rod. These factors may differ in every car, even in cars of the same model.
Clutch master cylinders vary in both bore diameter and stroke length, but are all made to conform to SAE (Society of Automotive Engineers) standard mounting dimensions as to the two bolt pattern and pilot diameter. Thus, an aluminum block, bolted to the firewall and machined to provide the standard SAE mounting, has become the commonly accepted cylinder installation adaptor. Should the push-rod line-of-action be misaligned when connected to the clutch pedal arm, a tapered shim is placed between the firewall sheet metal and the aluminum block. If even greater angular adjustment is required, the firewall face of the block may be machined to achieve the desired alignment. If not properly aligned, the bore, seals and piston will wear prematurely or the cylinder may be inoperable.
Master cylinder stroke length is also an issue in such an installation. There must be sufficient slave cylinder travel for positive clutch release action, yet overtravel is most undesirable. In order to limit the master cylinder output volume to only that needed by the slave cylinder, master cylinder stroke is usually limited by stopping pedal arm travel with a blocking piece placed under the clutch pedal, the thickness being adjusted by trial and error.